There has been developed a micro electro mechanical systems (MEMS) technology for forming a mechanical sensor such as a pressure sensor or an acceleration sensor, a miniaturized mechanical part such as a minute switch or a transducer, and a mechanical system, by using a micro-fabrication technology for forming a semiconductor integrated circuit device. The MEMS is broadly classified into a bulk MEMS that is formed by processing a silicon substrate itself to form micro components and a surface MEMS that is formed by repeating the deposition and patterning of thin films on a silicon substrate surface to form micro components.
As an acceleration sensor by the bulk MEMS, for example, there has been developed an element in which a weight formed by a handling layer and others of a silicon substrate or an SOI substrate is supported by a beam formed by an active layer, and a deformation of the beam due to the acceleration is detected on the basis of a piezo effect (refer to Japanese Patent Application Laid-Open Publication No. 2005-69946 (Patent Document 1) or U.S. Pat. No. 6,705,167B2 (Patent Document 2)).
As an acceleration sensor by the surface MEMS, for example, there has been developed an acceleration sensor in which a weight and a beam are formed by a polysilicon film having a thickness of about 2 μm to 4 μm, and a movement of the weight due to the acceleration is detected on the basis of a capacitance change between electrodes formed in both the weight and an outer frame (refer to Technical digest of IEEE electron devices meeting (2003) pp. 39.1.1 to 39.1.4 (Non-patent document 1)).
In these MEMS sensors, a mechanical deformation of a structure body due to an external force is converted into an electric signal as a piezo resistance change or a capacitance change, and in general, an output thereof is signal-processed by a semiconductor integrated circuit device (LSI: Large Scale Integration). As mentioned above, since the MEMS is utilized in combination with the LSI and the MEMS itself is manufactured by using a semiconductor manufacturing technology, there has been developed a technology for integrating both the elements on the same substrate in a monolithic manner.
For example, an acceleration sensor and a vibratory gyroscope using the weight made of a polysilicon film are integrated with an analogue circuit such as a capacitance-voltage conversion circuit and an operational amplifier. A sensor mechanism portion (arranged on the silicon substrate via a partial air gap) and the analogue circuit portion are arranged in different (adjacent) regions on a substrate surface. The sensor mechanism portion is sealed in a cavity which is wholly covered by a cover. For example, U.S. Pat. No. 5,760,455 (Patent Document 3) or U.S. Pat. No. 6,262,946B1 (Patent Document 4) has a description about a method of arranging pillar-like supports in the cavity portion.